Membranes, Methods of Making Membrane, and Methods of Separating Gases Using Membranes

ABSTRACT

Membrane, methods of making membranes, and methods of separating gases using membranes are provided. The membranes can include at least one hydrophilic polymer, at least one cross-linking agent, at least one base, and at least one amino compound. The methods of separating gases using membranes can include contacting a gas stream containing at least one of CO 2 , H 2 S, and HCl with one side of a nonporous and at least one of CO 2 , H 2 S, and HCl selectively permeable membrane such that at least one of CO 2 , H 2 S, and HCl is selectively transported through the membrane.

CROSS REFERENCE TO RELATED APPLICATIONS

This application claims priority to and any other benefit of U.S.Provisional Application Ser. No. 60/625,517, filed Nov. 5, 2004, whichis incorporated by reference in its entirety herein.

This invention was supported by US Department of Energy Grant No.DE-FC36-03AL68510. The government has certain rights in this invention.

BACKGROUND

There are numerous industrial processes that produce gas streamscontaining at least one of CO₂, H₂S, and HCl. It is often desirable toremove the CO₂, H₂S, and/or HCl from the other components of the gasstreams, such as H₂ and N₂. One technique used to selectively removeCO₂, H₂S, and/or HCl from streams is to absorb the CO₂, H₂S, and/or HClin an amine solution. Another technique used is to adsorb the CO₂, H₂S,and/or HCl on a molecular sieve. The scientific and industrial communityhas also used membranes to separate components in a process stream.There remains a need in the art for additional compositions, membranes,methods of making membranes, and methods of separating gases.

SUMMARY

In accordance with embodiments of the present invention, compositionsare provided. The compositions comprise at least one hydrophilicpolymer, at least one cross-linking agent, at least one base, and atleast one amino compound. The amino compound comprises at least one of apolyamine and a salt of aminoacid. The aminoacid salt is selected fromsalts having the formula:

wherein R₁, R₂, R₃, and R₄ are hydrogen or hydrocarbon groups havingfrom 1 to 4 carbon atoms, n is an integer ranging from 0 to 4, A^(m+) isa cation having a valence of 1 to 3 and an amine cation having theformula:

wherein R₅ and R₆ are hydrogen or hydrocarbon groups having from 1 to 4carbon atoms, R₇ is hydrogen or a hydrocarbon group having from 1 to 4carbon atoms or an alkyl amine of from 2 to 6 carbon atoms and 1 to 4nitrogen atoms, y is an integer ranging from 1 to 4, and m is an integerequal to the valence of the cation. The base is selected from potassiumhydroxide, sodium hydroxide, lithium hydroxide, triethylamine,N,N-dimethylaminopyridine, hexamethyltriethylenetetraamine, potassiumcarbonate, sodium carbonate, lithium carbonate, and combinationsthereof.

In accordance with embodiments of the present invention processes forseparating at least one of CO₂, H₂S, and HCl from a gas streamcontaining at least one of CO₂, H₂S, and HCl are provided. The processescomprise contacting a gas stream containing at least one of CO₂, H₂S,and HCl with one side of a nonporous and at least one of CO₂, H₂S, andHCl selectively permeable membrane and withdrawing from the obverse sideof the membrane a permeate containing of at least one of CO₂, H₂S, andHCl, wherein at least one of CO₂, H₂S, and HCl is selectively removedfrom the gaseous stream. The membrane comprises at least one hydrophilicpolymer, at least one cross-linking agent, at least one base, and atleast one amino compound. The amino compound comprises at least one of apolyamine and a salt of aminoacid, and the aminoacid salt is selectedfrom salts having the formula:

wherein R₁, R₂, R₃, and R₄ are hydrogen or hydrocarbon groups havingfrom 1 to 4 carbon atoms, n is an integer ranging from 0 to 4, A^(m+) isa cation having a valence of 1 to 3 and an amine cation having theformula:

wherein R₅ and R₆ are hydrogen or hydrocarbon groups having from 1 to 4carbon atoms, R₇ is hydrogen or a hydrocarbon group having from 1 to 4carbon atoms or an alkyl amine of from 2 to 6 carbon atoms and 1 to 4nitrogen atoms, y is an integer ranging from 1 to 4, and m is an integerequal to the valence of the cation. The base is selected from potassiumhydroxide, sodium hydroxide, lithium hydroxide, triethylamine,N,N-dimethylaminopyridine, hexamethyltriethylenetetraamine, potassiumcarbonate, sodium carbonate, lithium carbonate, and combinationsthereof. The at least one of CO₂, H₂S, and HCl is selectivelytransported through the membrane.

In accordance with further embodiments of the present invention methodsfor producing a nonporous membrane for separating at least one of CO₂,H₂S, and HCl from a gaseous stream containing at least one of CO₂, H₂S,and HCl are provided. The method comprises: forming a casting solutionof a solvent, at least one hydrophilic polymer, at least onecross-linking agent, at least one base, and at least one amino compound;casting the solution on a substrate; and evaporating the solvent suchthat a nonporous membrane is formed. The amino compound comprises atleast one of a polyamine and a salt of aminoacid. The aminoacid salt isselected from salts having the formula:

wherein R₁, R₂, R₃, and R₄ are hydrogen or hydrocarbon groups havingfrom 1 to 4 carbon atoms, n is an integer ranging from 0 to 4, A^(m+) isa cation having a valence of 1 to 3 and an amine cation having theformula:

wherein R₅ and R₆ are hydrogen or hydrocarbon groups having from 1 to 4carbon atoms, R₇ is hydrogen or a hydrocarbon group having from 1 to 4carbon atoms or an alkyl amine of from 2 to 6 carbon atoms and 1 to 4nitrogen atoms, y is an integer ranging from 1 to 4, and m is an integerequal to the valence of the cation. The base is selected from potassiumhydroxide, sodium hydroxide, lithium hydroxide, triethylamine,N,N-dimethylaminopyridine, hexamethyltriethylenetetraamine, potassiumcarbonate, sodium carbonate, lithium carbonate, and combinationsthereof.

In accordance with embodiments of the present invention methods forproducing a nonporous membrane for separating at least one of CO₂, H₂S,and HCl from a gaseous stream containing at least one of CO₂, H₂S, andHCl are provided. The methods comprise: forming a casting solution of asolvent, at least one hydrophilic polymer, at least one cross-linkingagent, and at least one base, and at least one amino compound; heatingthe solution; casting the solution on a substrate; and evaporating thesolvent such that a nonporous membrane is formed. The amino compoundcomprises at least one of a polyamine and a salt of aminoacid. Theaminoacid salt is selected from salts having the formula:

wherein R₁, R₂, R₃, and R₄ are hydrogen or hydrocarbon groups havingfrom 1 to 4 carbon atoms, n is an integer ranging from 0 to 4, A^(m+) isa cation having a valence of 1 to 3 and an amine cation having theformula:

wherein R₅ and R₆ are hydrogen or hydrocarbon groups having from 1 to 4carbon atoms, R₇ is hydrogen or a hydrocarbon group having from 1 to 4carbon atoms or are alkyl amine of from 2 to 6 carbon atoms and 1 to 4nitrogen atoms, y is an integer ranging from 1 to 4, and m is an integerequal to the valence of the cation. The base is selected from potassiumhydroxide, sodium hydroxide, lithium hydroxide, triethylamine,N,N-dimethylaminopyridine, hexamethyltriethylenetetraamine, potassiumcarbonate, sodium carbonate, lithium carbonate, and combinationsthereof;

It will be understood that the present invention is not limited to theembodiments described above.

DETAILED DESCRIPTION OF EMBODIMENTS OF THE INVENTION

The present invention will now be described with occasional reference tothe specific embodiments of the invention. This invention may, however,be embodied in different forms and should not be construed as limited tothe embodiments set forth herein. Rather, these embodiments are providedso that this disclosure will be thorough and complete, and will fullyconvey the scope of the invention to those skilled in the art.

Unless otherwise defined, all technical and scientific terms used hereinhave the same meaning as commonly understood by one of ordinary skill inthe art to which this invention belongs. The terminology used in thedescription of the invention herein is for describing particularembodiments only and is not intended to be limiting of the invention. Asused in the description of the invention and the appended claims, thesingular forms “a,” “an,” and “the” are intended to include the pluralforms as well, unless the context clearly indicates otherwise. Allpublications, patent applications, patents, and other referencesmentioned herein are incorporated by reference in their entirety.

Unless otherwise indicated, all numbers expressing quantities ofingredients, properties such as molecular weight, reaction conditions,and so forth as used in the description and claims are to be understoodas being modified in all instances by the term “about.” Accordingly,unless otherwise indicated, the numerical properties set forth in thefollowing description and claims are approximations that may varydepending on the desired properties sought to be obtained in embodimentsof the present invention. Notwithstanding that the numerical ranges andparameters setting forth the broad scope of the invention areapproximations, the numerical values set forth in the specific examplesare reported as precisely as possible. Any numerical values, however,inherently contain certain errors necessarily resulting from errorsfound in their respective measurements.

In accordance with embodiments of the present invention, compositionsare provided. The compositions comprise at least one of at least onehydrophilic polymer and at least one amino compound, at least onecross-linking agent, and at least one base. The amino compound comprisesat least one of a polyamine and a salt of aminoacid. The compositionscan be used to form nonporous membranes as discussed herein. Thecompositions can comprise any suitable amount of the hydrophilicpolymers. For example, the compositions can comprise from about 10 toabout 90 percent hydrophilic polymers by weight of the composition. Thecompositions can comprise any suitable amount of cross-linking agents.For example, the compositions can comprise about 1 to about 40 percentcross-linking agents by weight of the composition. The compositions cancomprise any suitable amount of bases. For example, the compositions cancomprise about 1 to about 40 percent bases by weight of the composition.The compositions can comprise any suitable amount of the aminocompounds. For example, the compositions can comprise from about 10 toabout 90 percent polyamines by weight of the composition and/or about 10to about 90 percent aminoacid salts by weight of the composition.

The hydrophilic polymers suitable for use in the present inventioninclude, but are not limited to, polyvinylalcohol, polyvinylacetate,polyethylene oxide, polyvinylpyrrolidone, polyacrylamine, and blends,and copolymers thereof. In one example, the hydrophilic polymercomprises polyvinylalcohol.

The polyamines suitable for use in the present invention include, butare not limited to, polyallylamine, polyethylenimine,poly-N-1,2-dimethylpropylallylamine, poly-N-methylallylamine,poly-N,N-dimethylallylamine, poly-2-vinylpiperidine, andpoly-4-vinylpiperidine, and blends and copolymers thereof. In oneexample, the hydrophilic polymer comprises polyallylamine. In anotherexample, the hydrophilic polymer comprises of polyethylenimine.

It will be understood that the compositions of the present invention caninclude either hydrophilic polymers, polyamines, or combinationsthereof. It will be further understood that the hydrophilic polymers andpolyamines may have any suitable weight average molecular weights. Forexample, the hydrophilic polymers and polyamines can have weight averagemolecular weights in the range of from about 15,000 to about 2,000,000and from about 50,000 to about 200,000. In another example, thepolyamines can comprise polyethylenimines with molecular weights in therange of from about 50,000 to about 100,000. In yet another example, thehydrophilic polymers can comprise polyvinylalcohols with molecularweights in the range of from about 50,000 to about 150,000.

The aminoacid salts in the compositions of the present invention areselected from salts having the formula:

wherein R₁, R₂, R₃, and R₄ are hydrogen or hydrocarbon groups havingfrom 1 to 4 carbon atoms, n is an integer ranging from 0 to 4, A^(m+) isa cation having a valence of 1 to 3 and an amine cation having theformula:

wherein R₅ and R₆ are hydrogen or hydrocarbon groups having from 1 to 4carbon atoms, R₇ is hydrogen or hydrocarbon groups having from 1 to 4carbon atoms or an alkyl amine of from 2 to 6 carbon atoms and 1 to 4nitrogen atoms, y is an integer ranging from 1 to 4, and m is an integerequal to the valence of the cation. It will be understood that thehydrocarbon can be saturated or unsaturated, branched or unbranched, andsubstituted or unsubstituted hydrocarbon, which may be substituted withheteroatoms in the hydrocarbon chain or at the end of the hydrocarbonchain. In one example, A^(m+) is a metal cation, and it can be selectedfrom Groups Ia, Ia, IIa, and VIII of the Periodic Table of Elements. Inanother example, A^(m+) can comprise lithium, aluminum, or iron. In yetanother example, the aminoacid salt can comprise aminoisobutyric acid-Ksalt, dimethylglycine-K salt, or dimethylglycine-Li salt.

The cross-linking agents suitable for use in the present inventioninclude, but are not limited to, formaldehyde, glutaraldehyde, maleicanhydride, glyoxal, divinylsulfone, toluenediisocyanate, trimethylolmelamine, terephthalatealdehyde, epichlorohydrin, vinyl acrylate, andcombinations thereof. In one example, the cross-linking agent comprisesformaldehyde, glutaraldehyde, or maleic anhydride.

The bases suitable for use in the present invention include, but are notlimited to, potassium hydroxide, sodium hydroxide, lithium hydroxide,triethylamine, N,N-dimethylaminopyridine,hexamethyltriethylenetetraamine, potassium carbonate, sodium carbonate,lithium carbonate, and combinations thereof. In one example, the basecomprises potassium hydroxide. It is believed, without intending to belimiting, that the base acts as a catalyst to catalyze the cross-linkingof hydrophilic polymers and polyamines in the compositions duringformation of the membranes. The base or bases remain in the membranesand constitute part of the membranes.

In accordance with embodiments of the present invention, thecompositions can be used to form nonporous membranes. It will beunderstood that the term “nonporous membrane” refers to a membranehaving a at least a portion that is substantially nonporous such that agas moves through the nonporous portion via diffusion rather thanseparation from a larger gas by pores. In one example, the membranes ofthe present invention are permeable to at least one of CO₂, H₂S, or HCl.In another example, the membranes of the present invention are CO₂selective versus hydrogen, nitrogen, or combinations thereof. In yetanother example, the membranes of the present invention are H₂Sselective versus hydrogen, nitrogen, or combinations thereof. In yetanother example, the membranes of the present invention are HClselective versus hydrogen, nitrogen, or combinations thereof. In afurther example, the membranes of the present invention are CO₂, H₂S,and HCl selective versus hydrogen, nitrogen, or combinations thereof. Inanother example, the membranes of the present invention may be used attemperatures of about 100° C. and greater than about 100° C. In yetanother example, the membranes of the present invention may be used attemperatures of from about 100° C., about 110° C., about 120° C., about130° C. about 140° C., about 150° C., about 160° C., about 170° C., andabout 180° C. In another example, the membranes of the present inventionmay be used at temperatures of less than about 100° C. The membranes canbe free standing membranes or composite membranes.

In accordance with embodiments of the present invention, methods forproducing nonporous membranes are provided. Formulations for forming themembranes are prepared using the compositions of the present invention.The formulations can be prepared by first forming a casting solution ofat least one of the hydrophilic polymer, the cross-linking agent, thebase, and the amino compound in a suitable solvent. One example of asuitable solvent is water. In one example, the amount of water employedwill be in the range of from about 50% to about 99%. The membranecomposition can then be recovered from the casting solution by removingthe solvent, for example, by allowing the solvent to evaporate.

In an alternative example, the casting solution can be used in forming anonporous membrane. The resulting casting solution or membranecomposition is formed into a nonporous membrane by using any suitabletechniques. For example, the casting solution can be cast onto asubstrate using any suitable techniques, and the solvent may beevaporated such that a nonporous membrane is formed on the substrate.Examples of suitable techniques include, but are not limited to, “knifecasting” or “dip casting”. Knife casting is a process in which a knifeis used to draw a polymer solution across a flat substrate to form athin film of a polymer solution of uniform thickness after which thesolvent of the polymer solution is evaporated, at ambient temperaturesor temperatures up to about 100° C. or higher, to yield a fabricatedmembrane. When, for example, a glass plate is used as the substrate, themembrane can then be removed from the substrate providing a freestanding polymer membrane. When, alternatively, the flat substrate usedis a non-selective porous support such as porouspolytetrafluoroethylene, the resulting membrane is a composite membranecomprising the selective membrane polymer and the support.

Dip casting is a process in which a polymer solution is contacted with anon-selective porous support. Then excess solution is permitted to drainfrom the support, and the solvent of the polymer solution is evaporatedat ambient or elevated temperatures as discussed above. The membranecomprises both the membrane polymer and the porous support. Themembranes of the present invention also may be shaped in the form ofhollow fibers, tubes, films, sheets, etc.

In other embodiments of the present invention, membranes formed fromcompositions containing a cross-linking agent can be formed in anysuitable manner. For example, the compositions of the present invention,including any solvent, can be heated at a temperature and for a timesufficient for cross-linking to occur. In one example, cross-linkingtemperatures in the range from about 80° C. to about 100° C. areemployed. In another example, cross-linking occurs in from about 1 toabout 72 hours. The resulting solution can be cast onto a substrate andthe solvent evaporated, as discussed above. In yet another example, ahigher degree of cross-linking for the cast membrane after solventremoval takes place at about 100° C. to about 180° C., and thecross-linking occurs in from about 1 to about 72 hours.

In other embodiments of the present invention, an additive may beincluded in the composition before forming a membrane to increase thewater retention ability of the membrane. Suitable additives include, butare not limited to, polystyrenesulfonic acid-K salt, polystyrenesulfonicacid-Na salt, polystyrenesulfonic acid-Li salt, sulfonatedpolyphenyleneoxides, alum, and combinations thereof. In one example, theadditive comprises polystyrenesulfonic acid-K salt.

In accordance with embodiments of the present invention, processes forseparating acid gases from a gas stream containing at least one acid gasare provided. The processes include contacting a gas stream containingat least one acid gas with one side of a nonporous acid gas selectivelypermeable membrane of the present invention, and withdrawing from theobverse side of the membrane a permeate containing at least one acidgas, wherein the acid gas is selectively removed from the gaseousstream. The permeate comprises the at least one acid gas in increasedconcentration relative to the feed stream. By “permeate” is meant thatportion of the feed stream which is withdrawn at the obverse or secondside of the membrane, exclusive of other fluids such as a sweep gas orliquid which may be present at the second side of the membrane. In oneexample, the acid gas is at least one of CO₂, H₂S, or HCl.

Without intending to be limiting, the membranes of the present inventionmay be used for the removal of at least one of CO₂, H₂S, or HCl fromgases including synthesis gases derived from fossil fuels that requirehydrogen purification for fuel cell, electricity generation, andhydrogenation applications, biogas for renewable energy, and natural gasfor commercial uses. The membranes can be used for removal of CO₂ fromflue gas containing nitrogen. It will be understood that the membranesof the present invention can be used for any other suitable gases.

The present invention will be better understood by reference to thefollowing examples which are offered by way of illustration notlimitation.

EXAMPLES

This invention is illustrated with the following non-limiting examples.In the examples, the separation factor (selectivity) for acid gas vs.hydrogen is expressed as follows:

${{Separation}\mspace{14mu} {Factor}} = \frac{\begin{matrix}{{Acid}\mspace{14mu} {{Gas}/}} \\{{Hydrogen}\mspace{14mu} {concentration}\mspace{14mu} {ratio}\mspace{14mu} {in}\mspace{14mu} {the}\mspace{14mu} {permeate}}\end{matrix}}{\begin{matrix}{{Acid}\mspace{14mu} {{Gas}/}} \\{{Hydrogen}\mspace{14mu} {concentration}\mspace{14mu} {ratio}\mspace{14mu} {in}\mspace{14mu} {the}\mspace{14mu} {retentate}}\end{matrix}}$

The retentate refers to the mixture on the feed side of the membranethat is rejected/retained by the membrane under the operatingconditions. Similarly, the separation factor for acid gas vs. nitrogenis expressed as follows:

${{Separation}\mspace{14mu} {Factor}} = \frac{\begin{matrix}{{Acid}\mspace{14mu} {{Gas}/}} \\{{Nitrogen}\mspace{14mu} {concentration}\mspace{14mu} {ratio}\mspace{14mu} {in}\mspace{14mu} {the}\mspace{14mu} {permeate}}\end{matrix}}{\begin{matrix}{{Acid}\mspace{14mu} {{Gas}/}} \\{{Nitrogen}\mspace{14mu} {concentration}\mspace{14mu} {ratio}\mspace{14mu} {in}\mspace{14mu} {the}\mspace{14mu} {retentate}}\end{matrix}}$

The flux, expressed in units of cm³ (STP)/(cm²·s), is related to thepermeability, expressed in units of Barrer (1 Barrer=10⁻¹⁰ cm³(STP)·cm/(cm²·s·cm Hg), in the following equation:

Flux=Permeability(p ₁ −p ₂)/l

where p₁ and p₂ are the acid gas (carbon dioxide, hydrogen sulfide, orhydrogen chloride) partial pressures in the retentate and permeatestreams, respectively, and l is the membrane thickness. The partialpressures are determined based on concentration measurements by gaschromatography and total pressure measurements by using pressure gauges.The flux is determined based on permeate concentration measurements bygas chromatography and permeate stream flow rate measurements by using aflow meter.

Example 1 Synthesis of 23.6 wt % Dimethylglycine-Li Salt, 23.6 wt %Polyethylenimine, 46.4 wt % (Polyvinylalcohol/Formaldehyde at 44/3.9 byWeight) and 6.4 wt % KOH Membrane

In order to prepare a polyvinylalcohol (PVA) solution, 13.15 g of PVAwas added to 88.54 g of water with stirring and heating at about 80° C.until a clear solution of the polymer was obtained. To this PVA solutionwere added an aqueous 37 wt % formaldehyde solution of 3.19 g (1.18 g offormaldehyde) and an aqueous KOH solution containing 2.01 g KOH and 8.72g water under stirring. The resulting solution was heated at 80-85° C.and maintained at this temperature range with stirring for 100 minutesto enhance the cross-linking of PVA with formaldehyde, catalyzed by KOH.In this solution, the KOH concentration was 1.74 wt %. ThePVA/formaldehyde weight ratio of 13.15/1.18, i.e., 44 (PVA monomermolecular weight)/3.9 (13% of formaldehyde molecular weight),corresponds to a maximum PVA cross-linking degree of 26%. To thissolution was added an aqueous polyethylenimine solution containing 7.38g polyethylenimine and 44.90 g water under stirring at 80-85° C.

Separately, an N,N-dimethylglycine-Li salt solution was prepared byadding 6.75 g (0.066 mole) of N,N-dimethylglycine and 2.77 g (0.066mole) of LiOHH₂O slowly to 9.10 g of water with stirring. This solutionwas added to the above PVA/formaldehyde/KOH solution under stirring at80-85° C. for about 30 minutes to obtain a clear, homogeneous solution.

The solution was then centrifuged at 5000 rpm while cooling for 30minutes. Following centrifugation, the membrane was knife-cast (with agap setting of 16 mils) onto a support of microporouspolytetrafluoroethylene. Water was allowed to evaporate from themembrane in a hood at ambient conditions overnight. The membrane wasthen heated in an oven at 120° C. for about 6 hours. The resultingmembrane comprised about 23.6 wt % dimethylglycine-Li salt, 23.6 wt %polyethylenimine, 46.4 wt % (polyvinylalcohol/formaldehyde at 44/3.9 byweight), and 6.4 wt % KOH. The membrane had a thickness of about 35microns (exclusive of the support).

Comparative Example A Synthesis of 25 wt % Dimethylglycine-Li Salt, 25wt % Polyethylenimine and 50 wt % (Polyvinylalcohol/Formaldehyde at44/3.9 by Weight) Membrane

Polyvinylalcohol (PVA) in an amount of 8.76 g was added to 58.82 g ofwater with stirring and heating at about 80° C. until a clear solutionof the polymer was obtained. To this PVA solution was added an aqueous37 wt % formaldehyde solution of 2.04 g (0.76 g of formaldehyde) understirring. The resulting solution was heated at about 95° C. andmaintained at this temperature with stirring for about 4 hours tothermally enhance the cross-linking of PVA with formaldehyde. To thissolution was added an aqueous polyethylenimine solution containing 4.58g polyethylenimine and 30.05 g water under stirring at 80-85° C.Separately, an N,N-dimethylglycine-Li salt solution was prepared byadding 4.50 g (0.044 mole) of N,N-dimethylglycine and 1.83 g (0.044mole) of LiOHH₂O slowly to 6.91 g of water with stirring. This solutionwas added to the above PVA/formaldehyde solution under stirring at80-85° C. for about 30 minutes to obtain a clear, homogeneous solution.

The solution was then centrifuged while cooling for about 30 minutes.Following centrifugation, the membrane was knife-cast (with a gapsetting of 18 mils) onto a support of microporouspolytetrafluoroethylene. Water was allowed to evaporate from themembrane in a hood at ambient conditions overnight. The membrane wasthen heated in an oven at 120° C. to give the resulting membrane. Thisresulting membrane comprised about 25 wt % dimethylglycine-Li salt, 25wt % polyethylenimine, and 50 wt % (polyvinylalcohol/formaldehyde at44/3.9 by weight). The membrane had a thickness of about 39 microns(exclusive of the support).

This membrane has the same ratios of dimethylglycine-Lisalt/polyethylenimine/polyvinylalcohol (PVA)/formaldehyde as those ofthe membrane of Example 1. However, the membrane of Example 2 did notcontain KOH whereas the membrane of Example 1 included KOH. The membraneof Example 2 was synthesized in the similar procedure taught in U.S.Pat. Nos. 5,611,843 and 6,099,621, and this membrane served as thecomparative example.

Permeation Measurement of Membrane of Example 1

In the measurement using a gas permeation apparatus to evaluate theseparation factor (selectivity) of carbon dioxide or hydrogen sulfidevs. hydrogen (or nitrogen) and the permeability of carbon dioxide orhydrogen sulfide, the membrane was placed in a permeation cellcomprising the first compartment for contacting a feed stream on theupstream side of the membrane and the second compartment for withdrawingthe permeate from the downstream side of the membrane. The activemembrane area in the cell was 45.6 cm². A feed gas comprising 40%hydrogen, 20% carbon dioxide, and 40% nitrogen (on the dry basis) undera total pressure of about 2 atm was contacted against the membrane at aflow rate of about 60 cm³/min (at ambient condition) in the gaspermeation apparatus. The permeate was swept by nitrogen under apressure of about 1 atm and a total flow rate of about 30 cm³/min forthe permeate/nitrogen stream. Both the feed and sweep streams werehumidified by injecting 0.03 ml/min of deionized water into each of thetwo streams prior to contacting the membrane. Each permeationmeasurement was carried out at an operating temperature.

For the membrane of Example 1 comprising about 23.6 wt %dimethylglycine-Li salt, 23.6 wt % polyethylenimine, 46.4 wt %(polyvinylalcohol/formaldehyde at 44/3.9 by weight), and 6.4 wt % KOH(with a thickness of 35 microns), the carbon dioxide/hydrogenselectivity results obtained were 1782, 277, and 285 at 80° C., 100° C.,and 110° C., respectively. The carbon dioxide permeability resultsobtained were 338, 49, and 50 Barrers at 80° C., 100° C., and 110° C.,respectively.

Comparative Permeation Measurement of Membrane of Comparative Example A

The membrane of Comparative Example A comprising about 25 wt %dimethylglycine-Li salt, 25 wt % polyethylenimine, and 50 wt %(polyvinylalcohol/formaldehyde at 44/3.9 by weight) was evaluated in thesame way as the membrane of Example 1 as described earlier in PermeationMeasurement of Membrane of Example 1. The carbon dioxide/hydrogenselectivity results obtained were 602, 453, 11 and 7 at 80° C., 90° C.,100° C., and 110° C., respectively. The selectivity result droppeddrastically at 100° C., indicating excessive membrane swelling andinsufficient thermal stability at the temperatures greater than 100° C.

In comparison of the carbon dioxide/hydrogen selectivity results betweenthe membranes of Examples 1 and Comparative Example A, the membrane ofExample 1 had better results than the membrane of Comparative Example A,particularly for temperatures greater than 100° C. At temperaturesgreater than 100° C., the membrane of Comparative Example A had a carbondioxide/hydrogen selectivity of 11 or lower. However, the membrane ofExamples 1 of the present invention still had a carbon dioxide/hydrogenselectivity of 277 or higher at 100-110° C. Thus, this membrane of thepresent invention that was synthesized with the base (KOH) outperformedthe membrane that was synthesized without this base.

Example 2 Synthesis of 23.7 wt % Dimethylglycine-K Salt, 4.7 wt %Polystyrenesulfonic Acid-K Salt, 54 wt % (Polyvinylalcohol/Formaldehydeat 44/9 by Weight) and 17.6 wt % KOH Membrane

To 54.06 g of water was added 8.823 g of polyvinylalcohol (PVA) withstirring and heating at about 80° C. until a clear solution of thepolymer was obtained. Separately, a polystyrenesulfonic acid-K saltsolution was prepared by adding 2.563 g of 30% polystyrenesulfonic acidand 0.232 g of KOH slowly to 1.293 g of water with stirring. Thissolution was adjusted using KOH to have a pH of 7 and contained 0.927 gof polystyrenesulfonic acid-K salt. This solution was added to the PVAsolution with stirring at about 80° C. To the PVA solution with thepolystyrenesulfonic acid-K salt were added an aqueous 37 wt %formaldehyde solution of 4.869 g (1.802 g of formaldehyde) and anaqueous KOH solution containing 3.460 g KOH and 3.341 g water understirring. The resulting solution was heated at about 80° C. andmaintained at this temperature with stirring for 6 hours to enhance thecross-linking of PVA with formaldehyde, catalyzed by KOH. In thissolution, the KOH concentration was about 4.4 wt %. The PVA/formaldehydeweight ratio of 8.823/1.802, i.e., 44 (PVA monomer molecular weight)/9(30% of formaldehyde molecular weight), corresponds to a maximum PVAcross-linking degree of 60%. Separately, an N,N-dimethylglycine-K saltsolution was prepared by adding 3.369 g (0.033 mole) ofN,N-dimethylglycine and 1.898 g (0.033 mole) of KOH slowly to 3.015 g ofwater with stirring. This solution was added to the abovePVA/formaldehyde/KOH/polystyrenesulfonic acid-K salt solution understirring at about 80° C. for about 30 minutes to obtain a clear,homogeneous solution.

The solution was then centrifuged at 8000 rpm while cooling for 12minutes. Following centrifugation, the membrane was knife-cast (with agap setting of 11 mils) onto a support of microporouspolytetrafluoroethylene. Water was allowed to evaporate from themembrane in a hood at ambient conditions overnight. The membrane wasthen heated in an oven at 120° C. for about 6 hours. The resultingmembrane comprised about 23.7 wt % dimethylglycine-K Salt, 4.7 wt %polystyrenesulfonic acid-K salt, 54.0 wt %(polyvinylalcohol/formaldehyde at 44/9 by weight), and 17.6 wt % KOH.The membrane had a thickness of about 33 microns (exclusive of thesupport).

Permeation Measurement of Membrane of Example 2

The membrane of Example 2 comprising about 23.7 wt % dimethylglycine-KSalt, 4.7 wt % polystyrenesulfonic acid-K salt, 54.0 wt %(polyvinylalcohol/formaldehyde at 44/9 by weight), and 17.6 wt % KOH(with a thickness of about 33 microns) was evaluated in the same waydescribed in Permeation Measurement of Membrane of Example 1. The carbondioxide/hydrogen selectivity and carbon dioxide permeability resultsobtained at 120-170° C. are shown in Table 1.

TABLE 1 The carbon dioxide/hydrogen selectivity and carbon dioxidepermeability results for the membrane of Example 2. Temperature CarbonDioxide/Hydrogen Carbon Dioxide Permeability (° C.) Selectivity (Barrer)120 311 1287 140 200 722 150 84 318 160 42 202 170 25 172

As shown in this table, this membrane possessed a high selectivity ofcarbon dioxide vs. hydrogen as well as high carbon dioxide permeability.Thus, this membrane is useful for the removal of carbon dioxide from thehydrogen-containing synthesis gas comprising this impurity to increasethe concentration of hydrogen for hydrogen purification and enhancement.

Example 3 Synthesis of 27.2 wt % Aminoisobutyric Acid-K Salt, 10.1 wt %Polyallylamine, 45.9 wt % (Polyvinylalcohol/Formaldehyde at 44/9 byWeight) and 16.8 wt % KOH Membrane

To 50.588 g of water was added 8.800 g of polyvinylalcohol (PVA) withstirring and heating at about 80° C. until a clear solution of thepolymer was obtained. To this PVA solution were added an aqueous 37 wt %formaldehyde solution of 4.868 g (1.801 g of formaldehyde) and anaqueous KOH solution containing 3.880 g KOH and 3.933 g water understirring. The resulting solution was heated at about 80° C. andmaintained at this temperature with stirring for 6 hours to enhance thecross-linking of PVA with formaldehyde, catalyzed by KOH. In thissolution, the KOH concentration was about 5.4 wt %. The PVA/formaldehydeweight ratio of 8.800/1.801, i.e., 44 (PVA monomer molecular weight)/9(30% of formaldehyde molecular weight), corresponds to a maximum PVAcross-linking degree of 60%. Separately, an aminoisobutyric acid-K saltsolution was prepared by adding 4.480 g (0.044 mole) of aminoisobutyricacid and 2.580 g (0.046 mole) of KOH slowly to 5.284 g of water withstirring. A polyallylamine solution was prepared by adding 2.327 gpolyallylamine in 15.234 g water. To the above PVA/formaldehyde/KOHsolution were added the aminoisobutyric acid-K salt solution and thepolyallylamine solution under stirring at about 80° C. for about 30minutes to obtain a clear, homogeneous solution.

The solution was then centrifuged at 8000 rpm while cooling for 10minutes. Following centrifugation, the membrane was knife-cast (with agap setting of 11 mils) onto a support of microporouspolytetrafluoroethylene. Water was allowed to evaporate from themembrane in a hood at ambient conditions overnight. The membrane wasthen heated in an oven at 120° C. for about 6 hours. The resultingmembrane comprised about 27.2 wt % aminoisobutyric acid-K salt, 10.1 wt% polyallylamine, 45.9 wt % (polyvinylalcohol/formaldehyde at 44/9 byweight), and 16.8 wt % KOH. The membrane had a thickness of about 26microns (exclusive of the support).

Permeation Measurement of Membrane of Example 3

The membrane of Example 3 comprising about 27.2 wt % aminoisobutyricacid-K salt, 10.1 wt % polyallylamine, 45.9 wt %(polyvinylalcohol/formaldehyde at 44/9 by weight), and 16.8 wt % KOH(with a thickness of about 26 microns) was evaluated in the similar waydescribed in Permeation Measurement of Membrane of Example 1 except forthe water injection rate to the feed gas. The water injection rate tothe feed gas was 0.03 ml/min at 120-130° C., 0.06 ml/min at 140° C.,0.09 ml/min at 150° C., and 0.12 ml/min at 160-180° C. The carbondioxide/hydrogen selectivity and carbon dioxide permeability resultsobtained at 120-180° C. are shown in Table 2.

TABLE 2 The carbon dioxide/hydrogen selectivity and carbon dioxidepermeability results for the membrane of Example 3. Temperature CarbonDioxide/Hydrogen Carbon Dioxide Permeability (° C.) Selectivity (Barrer)120 262 6196 130 193 3922 140 161 4463 150 80 3651 160 67 3039 170 532241 180 10.1 1941

As shown in this table, this membrane exhibited a very high selectivityof carbon dioxide vs. hydrogen as well as very high carbon dioxidepermeability. Thus, this membrane is very useful for the removal ofcarbon dioxide from the hydrogen-containing synthesis gas comprisingthis impurity to increase the concentration of hydrogen for hydrogenpurification and enhancement.

Example 4 Synthesis of 19.6 wt % Aminoisobutyric Acid-K Salt, 9.8 wt %Polyallylamine, 52.5 wt % (Polyvinylalcohol/Formaldehyde at 44/15 byWeight) and 18.1 wt % KOH Membrane

To 48.361 g of water was added 8.805 g of polyvinylalcohol (PVA) withstirring and heating at about 80° C. until a clear solution of thepolymer was obtained. To this PVA solution were added an aqueous 37 wt %formaldehyde solution of 8.123 g (3.006 g of formaldehyde) and anaqueous KOH solution containing 4.072 g KOH and 4.331 g water understirring. The resulting solution was heated at about 80° C. andmaintained at this temperature with stirring for 23.5 hours to enhancethe cross-linking of PVA with formaldehyde, catalyzed by KOH. In thissolution, the KOH concentration was about 5.5 wt %. The PVA/formaldehydeweight ratio of 8.805/3.006, i.e., 44 (PVA monomer molecular weight)/15(50% of formaldehyde molecular weight), corresponds to a maximum PVAcross-linking degree of 100%. Separately, an aminoisobutyric acid-K saltsolution was prepared by adding 3.123 g of aminoisobutyric acid and1.833 g of KOH slowly to 4.432 g of water with stirring. Apolyallylamine solution was prepared by adding 2.206 g polyallylamine in10.325 g water. To the above PVA/formaldehyde/KOH solution were addedthe aminoisobutyric acid-K salt solution and the polyallylamine solutionunder stirring at about 80° C. for about 10 minutes to obtain a clear,homogeneous solution.

The solution was then centrifuged at 8000 rpm while cooling for 8minutes. Following centrifugation, the membrane was knife-cast onto asupport of microporous polytetrafluoroethylene. Water was allowed toevaporate from the membrane in a hood at ambient conditions overnight.The membrane was then heated in an oven at 120° C. for about 6 hours.The resulting membrane comprised about 19.6 wt % aminoisobutyric acid-Ksalt, 9.8 wt % polyallylamine, 52.5 wt % (polyvinylalcohol/formaldehydeat 44/15 by weight), and 18.1 wt % KOH. The membrane had a thickness ofabout 51 microns (exclusive of the support).

Permeation Measurement of Membrane of Example 4

The membrane of Example 4 comprising about 19.6 wt % aminoisobutyricacid-K salt, 9.8 wt % polyallylamine, 52.5 wt %(polyvinylalcohol/formaldehyde at 44/15 by weight), and 18.1 wt % KOH(with a thickness of about 51 microns) was evaluated in the similar waydescribed in the Permeation Measurement of Membrane of Example 1, exceptfor the feed gas and the water injection rates. The feed gas contained50 ppm hydrogen sulfide in 17% carbon dioxide, 1% carbon monoxide, 45%hydrogen, and 37% nitrogen (on the dry basis). The water injection rateto sweep gas was maintained constant at 0.27 ml/min. However, the waterinjection rate to the feed gas was 0.075 ml/min at 110° C., 0.09 ml/minat 120-130° C., and 0.12 ml/min at 140° C. Table 3 shows the carbondioxide/hydrogen, hydrogen sulfide/hydrogen and carbon dioxide/nitrogenselectivities, and carbon dioxide and hydrogen sulfide permeabilitiesobtained at 110-140° C.

TABLE 3 The carbon dioxide/hydrogen, hydrogen sulfide/hydrogen andcarbon dioxide/nitrogen selectivities, and carbon dioxide and hydrogensulfide permeabilities for the membrane of Example 4. Tem- CarbonHydrogen Carbon Carbon Hydrogen per- Dioxide/ Sulfide/ Dioxide/ DioxideSulfide ature Hydrogen Hydrogen Nitrogen Permeability Permeability (°C.) Selectivity Selectivity Selectivity (Barrer) (Barrer) 110 170 5731218 8278 28437 120 234 685 1439 8847 21780 130 230 673 1341 7555 21233140 125 298 440 4772 10015

As shown in this table, this membrane had very high selectivities ofcarbon dioxide and hydrogen sulfide vs. hydrogen as well as very highcarbon dioxide and hydrogen sulfide permeabilities. In other words, thismembrane is very useful for the removal of both carbon dioxide andhydrogen sulfide from the hydrogen-containing synthesis gas comprisingthese impurities. Also shown in this table, this membrane had very highselectivity of carbon dioxide vs. nitrogen. Thus, this membrane is alsouseful for the removal of the greenhouse gas, carbon dioxide, from thenitrogen-containing flue gas.

The present invention should not be considered limited to the specificexamples described above, but rather should be understood to cover allaspects of the invention. Various modifications, equivalent processes,as well as numerous structures and devices to which the presentinvention may be applicable will be readily apparent to those of skillin the art.

It will be understood that various changes may be made without departingfrom the scope of the invention, which is not to be considered limitedto what is described in the specification.

1. A composition comprising: at least one hydrophilic polymer, at leastone cross-linking agent, at least one base, and at least one aminocompound, wherein the amino compound comprises at least one of apolyamine and a salt of aminoacid, and wherein the aminoacid salt isselected from salts having the formula:

wherein R₁, R₂, R₃, and R₄ are hydrogen or hydrocarbon groups havingfrom 1 to 4 carbon atoms, n is an integer ranging from 0 to 4, A^(m+) isa cation having a valence of 1 to 3 and an amine cation having theformula:

wherein R₅ and R₆ are hydrogen or hydrocarbon groups having from 1 to 4carbon atoms, R₇ is hydrogen or a hydrocarbon group having from 1 to 4carbon atoms or an alkyl amine of from 2 to 6 carbon atoms and 1 to 4nitrogen atoms, y is an integer ranging from 1 to 4, and m is an integerequal to the valence of the cation; and wherein said base is selectedfrom potassium hydroxide, sodium hydroxide, lithium hydroxide,triethylamine, N,N-dimethylaminopyridine,hexamethyltriethylenetetraamine, potassium carbonate, sodium carbonate,lithium carbonate, and combinations thereof.
 2. The composition asclaimed in claim 1 wherein said at least one amino compound comprises apolyamine selected from polyallylamine, polyethylenimine,poly-N-1,2-dimethylpropylallylamine, poly-N-methylallylamine,poly-N,N-dimethylallylamine, poly-2-vinylpiperidine, andpoly-4-vinylpiperidine, and blends and copolymers thereof.
 3. Thecomposition as claimed in claim 1 wherein said at least one aminocompound comprises polyallylamine.
 4. The composition as claimed inclaim 1 wherein said at least one amino compound comprisespolyethylenimine.
 5. The composition as claimed in claim 1 wherein saidat least one hydrophilic polymer is selected from polyvinylalcohol,polyvinylacetate, polyethylene oxide, polyvinylpyrrolidone,polyacrylamine, and blends, and copolymers thereof.
 6. The compositionas claimed in claim 1 wherein said at least one hydrophilic polymercomprises polyvinylalcohol.
 7. The composition as claimed in claim 6wherein said at least one amino compound comprises polyallylamine. 8.The composition as claimed in claim 6 wherein said at least one aminocompound comprises polyethylenimine.
 9. The composition as claimed inclaim 1 wherein A^(m+) is a metal cation selected from Groups Ia, IIa,IIIa, and VIII of the Periodic Table of Elements.
 10. The composition asclaimed in claim 9 wherein said at least one amino compound comprisesaminoisobutyric acid-K salt.
 11. The composition as claimed in claim 9wherein said at least one amino compound comprises dimethylglycine-Ksalt.
 12. The composition as claimed in claim 9 wherein said at leastone amino compound comprises dimethylglycine-Li salt.
 13. Thecomposition as claimed in claim 1 wherein said at least onecross-linking agent is selected from formaldehyde, glutaraldehyde,maleic anhydride, glyoxal, divinylsulfone, toluenediisocyanate,trimethylol melamine, terephthalatealdehyde, epichlorohydrin, vinylacrylate, and combinations thereof.
 14. The composition as claimed inclaim 13 wherein said at least one cross-linking agent comprisesformaldehyde.
 15. The composition as claimed in claim 1 wherein said atleast one cross-linking agent comprises from about 1 to about 40 percentby weight of said composition.
 16. The composition as claimed in claim 1wherein said at least one base comprises from about 1 to about 40percent by weight of said composition.
 17. The composition as claimed inclaim 1 wherein said at least one hydrophilic polymer comprises fromabout 10 to about 90 percent by weight of said composition.
 18. Thecomposition as claimed in claim 1 wherein said at least one aminocompound comprises from about 10 to about 90 percent by weight of saidcomposition.
 19. The composition as claimed in claim 1 wherein said atleast one base comprises potassium hydroxide.
 20. A nonporous membraneformed from the composition of claim
 1. 21. A process for separating atleast one of CO₂, H₂S, and HCl from a gas stream containing at least oneof CO₂, H₂S, and HCl, comprising: contacting a gas stream containing atleast one of CO₂, H₂S, and HCl with one side of a nonporous and at leastone of CO₂, H₂S, and HCl selectively permeable membrane comprising: atleast one hydrophilic polymer, at least one cross-linking agent, atleast one base, and at least one amino compound, wherein said aminocompound comprises at least one of a polyamine and a salt of aminoacid,and wherein the aminoacid salt is selected from salts having theformula:

wherein R₁, R₂, R₃, and R₄ are hydrogen or hydrocarbon groups havingfrom 1 to 4 carbon atoms, n is an integer ranging from 0 to 4, A^(m+) isa cation having a valence of 1 to 3 and an amine cation having theformula:

wherein R₅ and R₆ are hydrogen or hydrocarbon groups having from 1 to 4carbon atoms, R₇ is hydrogen or a hydrocarbon group having from 1 to 4carbon atoms or an alkyl amine of from 2 to 6 carbon atoms and 1 to 4nitrogen atoms, y is an integer ranging from 1 to 4, and m is an integerequal to the valence of the cation; wherein said base is selected frompotassium hydroxide, sodium hydroxide, lithium hydroxide, triethylamine,N,N-dimethylaminopyridine, hexamethyltriethylenetetraamine, potassiumcarbonate, sodium carbonate, lithium carbonate, and combinationsthereof; wherein at least one of CO₂, H₂S, and HCl is selectivelytransported through the membrane; and withdrawing from the obverse sideof said membrane a permeate containing of at least one of CO₂, H₂S, andHCl, wherein at least one of CO₂, H₂S, and HCl is selectively removedfrom said gaseous stream.
 22. The process as claimed in claim 21 whereinsaid at least one amino compound comprises a polyamine selected frompolyallylamine, polyethylenimine, poly-N-1,2-dimethylpropylallylamine,poly-N-methylallylamine, poly-N,N-dimethylallylamine,poly-2-vinylpiperidine, and poly-4-vinylpiperidine, and blends andcopolymers thereof.
 23. The process as claimed in claim 22 wherein saidat least one amino compound comprises polyallylamine.
 24. The process asclaimed in claim 22 wherein said at least one amino compound comprisespolyethylenimine.
 25. The process as claimed in claim 21 wherein said atleast one hydrophilic polymer is selected from polyvinylalcohol,polyvinylacetate, polyethylene oxide, polyvinylpyrrolidone,polyacrylamine, and blends, and copolymers thereof.
 26. The process asclaimed in claim 21 wherein said at least one hydrophilic polymercomprises polyvinylalcohol.
 27. The process as claimed in claim 21wherein said at least one amino compound comprises aminoisobutyricacid-K salt.
 28. The process as claimed in claim 21 wherein at least oneamino compound comprises dimethylglycine-K salt.
 29. The process asclaimed in claim 21 wherein said amino compound comprisesdimethylglycine-Li salt.
 30. The process as claimed in claim 21 whereinat least one cross-linking agent is selected from formaldehyde,glutaraldehyde, maleic anhydride, glyoxal, divinylsulfone,toluenediisocyanate, trimethylol melamine, terephthalatealdehyde,epichlorohydrin, vinyl acrylate, and combinations thereof.
 31. Theprocess as claimed in claim 21 wherein at least one cross-linking agentcomprises formaldehyde.
 32. A method for producing a nonporous membranefor separating at least one of CO₂, H₂S, and HCl from a gaseous streamcontaining at least one of CO₂, H₂S, and HCl, the method comprising:forming a casting solution of a solvent, at least one hydrophilicpolymer, at least one cross-linking agent, at least one base, and atleast one amino compound, wherein the amino compound comprises at leastone of a polyamine and a salt of aminoacid, and wherein the aminoacidsalt is selected from salts having the formula:

wherein R₁, R₂, R₃, and R₄ are hydrogen or hydrocarbon groups havingfrom 1 to 4 carbon atoms, n is an integer ranging from 0 to 4, A^(m+) isa cation having a valence of 1 to 3 and an amine cation having theformula:

wherein R₅ and R₆ are hydrogen or hydrocarbon groups having from 1 to 4carbon atoms, R₇ is hydrogen or a hydrocarbon group having from 1 to 4carbon atoms or an alkyl amine of from 2 to 6 carbon atoms and 1 to 4nitrogen atoms, y is an integer ranging from 1 to 4, and m is an integerequal to the valence of the cation, and wherein said base is selectedfrom potassium hydroxide, sodium hydroxide, lithium hydroxide,triethylamine, N,N-dimethylaminopyridine,hexamethyltriethylenetetraamine, potassium carbonate, sodium carbonate,lithium carbonate, and combinations thereof; casting the solution on asubstrate; and evaporating the solvent such that a nonporous membrane isformed.
 33. The method as claimed in claim 32 wherein said at least oneamino compound comprises a polyamine selected from polyallylamine,polyethylenimine, poly-N-1,2-dimethylpropylallylamine,poly-N-methylallylamine, poly-N,N-dimethylallylamine,poly-2-vinylpiperidine, and poly-4-vinylpiperidine, and blends andcopolymers thereof.
 34. The method as claimed in claim 33 wherein saidpolyamine comprises polyallylamine.
 35. The method as claimed in claim33 wherein said polyamine comprises polyethylenimine.
 36. The method asclaimed in claim 32 wherein said at least one hydrophilic polymer isselected from polyvinylalcohol, polyvinylacetate, polyethylene oxide,polyvinylpyrrolidone, polyacrylamine, and blends, and copolymersthereof.
 37. The method as claimed in claim 32 wherein said at least onehydrophilic polymer comprises polyvinylalcohol.
 38. The method asclaimed in claim 32 wherein said at least one amino compound comprisesaminoisobutyric acid-K salt.
 39. The method as claimed in claim 32wherein said at least one amino compound comprises dimethylglycine-Ksalt.
 40. The method as claimed in claim 32 wherein said at least oneamino compound comprises dimethylglycine-Li salt.
 41. The method asclaimed in claim 32 wherein said at least one cross-linking agent isselected from formaldehyde, glutaraldehyde, maleic anhydride, glyoxal,divinylsulfone, toluenediisocyanate, trimethylol melamine,terephthalatealdehyde, epichlorohydrin, vinyl acrylate, and combinationsthereof.
 42. The method as claimed in claim 32 wherein said at least onecross-linking agent comprises formaldehyde.
 43. The method as claimed inclaim 32 further comprising the step of heating the solution prior tothe step of casting the solution on a substrate.